Chemistry Study Material (5th & 6th Semester) – Quantum Chemistry free pdf university of kashmir


Black-body radiation
Black body also known as an ideal emitter is an object capable of emitting and absorbing
all frequencies of radiation uniformly. A hot object emits always electromagnetic radiation and at
high temperatures, an appreciable proportion of the radiation is in the visible region of the
spectrum, and a higher proportion of short-wavelength blue light is generated as the temperature
is raised. This behaviour is seen when a heated iron bar glowing red hot becomes white hot when
heated further. The dependence is illustrated in Fig. 1, which shows how the energy output varies
with wavelength at several temperatures. Energy density increases in the region of shorter
wavelengths as the temperature is raised, and the peak shifts to shorter wavelengths. The total
energy density (the area under the curve) increases as the temperature is increased.A good approximation to a black body is a pinhole in an empty container maintained at a
constant temperature, because any radiation leaking out of the hole has been absorbed and reemitted inside so many times that it has come to thermal equilibrium with the walls as shown in The explanation of black-body radiation was theoretically given by physicist Lord Rayleigh who
studied it from a classical viewpoint, and thought of the electromagnetic field as a collection of
oscillators of all possible frequencies. He regarded the presence of radiation of frequency ν (and
therefore of wavelength λ = c/ν) as signifying that the electromagnetic oscillator of that
frequency had been excited. Rayleigh used the equipartition principle to calculate the average
energy of each oscillator as kT. Then, with minor help from James Jeans, he arrived at the
Rayleigh–Jeans law as

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